1. Field of the Invention
The present invention relates to the field of medical instrumentation, specifically the use of electronic storage devices for storing and retrieving data relating to, inter alia, particular instruments or patients.
2. Description of Related Technology
The ability to readily measure various physiologic parameters associated with a living subject, such as arterial blood pressure or ECG, is often critical to providing effective care to such subjects. Typically, under the prior art, measurement of such parameters is accomplished using a system comprising a host device such as a portable or semi-portable monitoring station that is used in conjunction with a replaceable/disposable probe or sensor assembly, the latter being in direct contact with the subject and measuring the physical parameter (or related parameters) of interest. Such replaceable and disposable sensor assemblies are highly desirable from the standpoint that the risk of transfer of bacterial or other contamination from one patient to the next is significantly mitigated; the portion of the sensor assembly (or for that matter entire assembly) in contact with a given subject is replaced before use on another subject.
However, despite the mitigated risk of contamination, the use of such prior art disposable sensors also includes certain risks. One such risk relates to the potential re- use of what are meant to be single-use only components. Inherently, individuals or health care providers may attempt to re-use such single use components if there is no seeming degradation of the component or perceived threat of contamination. However, in the case of certain devices, the degradation of the component may be insidious and not immediately perceptible to the user. For example, the offset (i.e., difference of voltage generated by the device at certain prescribed conditions) associated with an elastomer-coated pressure transducer used in a non-invasive blood pressure monitoring device may change progressively in small increments over time due to swelling of the elastomer coating resulting from exposure to certain chemical substances. This variation in offset manifests itself as a change in the ultimate blood pressure reading obtained using the device, thereby reducing its accuracy. Hence, the readings obtained using the instrument may appear to be reasonable or correct, but in fact will incorporate increasing amounts of error from the true value of the parameter, which may significantly impact the treatment ultimately provided to the subject. Hence, what is needed is an approach wherein any such degradable or single use components are reliably replaced at the necessary interval such that performance does not appreciably degrade.
A related issue concerns the re-use of such devices on different patients. Specifically, if the xe2x80x9csingle usexe2x80x9d components are perceived by the user not to degrade rapidly, the user may be tempted to use the device (including the single use transducer(s)) on several different patients. Aside from the aforementioned performance issues, such repeated use may be hazardous from a contamination standpoint, as previously discussed. Ideally, portions of the device capable of transmitting bacterial, viral, or other deleterious agents are disposed of and replaced prior to use on another patient.
Another risk concerns the use of third party or non-compliant sensors with the host device of the original equipment manufacturer (OEM). While such third party sensors may ostensibly be manufactured to the design specifications and requirements of the OEM, in many cases they are not, which can result in readings obtained using the system which are less than accurate or even wholly non-representative of the parameter being measured. Even OEM supplied disposable sensors may have defects. Another troubling aspect is the fact that the caregiver or health care professional who is provided with such disposable sensors may have no means by which to verify the quality or acceptability of a given replaceable sensor, and therefore the accuracy of any reading they may obtain using that sensor may be called into question. Hence, even if the majority of sensors within a given lot obtained from the third party manufacturer are acceptable in terms of performance, the caregiver often has no way of knowing whether the next replacement sensor they use will perform as designed or intended by the OEM and yield representative results. In the ideal case, the quality of each individual replacement sensor would be determined by the host system prior to use (such as when the new replacement sensor is first installed on the host), and the caregiver apprised of the results of this determination.
The calibration of the replaceable/disposable sensor, whether OEM or otherwise, and the host system must also be considered. Under the prior art approach, calibration is most often performed on the system as a whole at a discrete point in time, and is generally not performed before each use of the device after a new sensor or probe has been installed. Hence, the calibration of the host system and replaceable sensor as a whole is not specific to each given sensor, but rather to a xe2x80x9cnominalxe2x80x9d sensor (i.e., the one in place in the system when the calibration was performed). For example, the system may be calibrated before first use, and then periodically thereafter at predetermined intervals, or at the occurrence of a given condition. Under this approach, changes in the physical operating characteristics of the host system may result in changes in the calibration over time. Due to any number of intrinsic or external factors, the device may xe2x80x9cdriftxe2x80x9d between calibrations, such that a reading taken with the device immediately following calibration may be substantially different from that obtained using the same device and identical conditions immediately before the next calibration.
Additionally, due to manufacturing tolerances and variations, the performance of each individual replaceable sensor may vary significantly from other similar devices, as previously described. Such variations are generally accounted for by the OEM by specifying a maximum allowable tolerances or variances for certain critical parameters associated with the sensors; if these tolerances/variances are met for a given replaceable sensor, then the accuracy of the system as a whole will fall within a certain (acceptable) tolerance as well. Ideally, however, the system would be calibrated specifically to each individual replaceable sensor immediately prior to use, a capability which is not present in prior art disposable medical devices.
Another concern relates to the potential for surreptitious alteration of data stored by an instrument prior to or during operation. As with many other types of devices, the ability to make a device xe2x80x9ctamperproofxe2x80x9d is of significant importance, in that this provides the caregiver and subject with additional assurance that the disposable sensor in use is the correct type of sensor for the host system, that the sensor assembly and host system are properly calibrated, and that the disposable sensor has not been used on other subjects.
Lastly, it is recognized that prior art measurement systems do not include the facility for evaluating the accuracy of a given measurement or host/sensor combination after readings have been taken. Many systems are capable of storing data relating to a measurement obtained from a subject in terms of the estimated value(s) derived by the system, yet none of which the Assignee hereof is aware allow for the retrieval of data specific to a given sensor or permit the system operator to evaluate the performance (and accuracy) of the system historically. Such information is of great potential utility in the medical field, especially with relation to medical malpractice litigation, by enabling the caregiver or OEM to reconstruct the operation of their equipment to demonstrate that a given measurement obtained using a given sensor and host unit was in fact accurate, that the disposable sensor had been replaced prior to use on the patient, and the like. The availability of this information may also produce the added benefit of reduced medical malpractice insurance premiums for facilities using such systems, since the potential for fraudulent claims relating to the system is reduced.
Based on the foregoing, what is needed is an apparatus and associated method useful for measuring one or more physiologic parameters associated with a living subject wherein any degradable or single use components associated with the apparatus may be easily and reliably replaced so as to ensure that (i) the accuracy of the system and any measurements resulting there from do not degrade; (ii) cross-contamination between subjects does not occur; and (iii) the operating history of the replaced components and system as a whole may be subsequently retrieved for analysis.
The present invention satisfies the aforementioned needs by an improved apparatus and method for monitoring the physiologic parameters, such as for example arterial blood pressure, of a living subject.
In a first aspect of the invention, an improved sensor assembly incorporating an electronic storage element is disclosed. In a first embodiment, the device comprises one or more ultrasonic transducers and a removable (and disposable) pressure transducer, the latter further including a storage device in the form of an electrically erasable programmable read-only memory (EEPROM) capable of storing data and information relating to the operation of the sensor assembly, host system, and patient. The sensor EEPROM includes a variety of information relating to the manufacture, run time, calibration, and operation of the pressure transducer, as well as application specific data such as patient or health care facility identification. Portions of the data are encrypted to prevent tampering. Furthermore, the host system is programmed such that the sensor assembly will be rejected and rendered unusable by the host if certain portions of the aforementioned data do not meet specific criteria. In this fashion, system operational integrity, maintainability, and patient safety are significantly enhanced. In a second embodiment, one or more additional storage devices (e.g., EEPROMs) are included within the host system to permit the storage of data relating to the system and a variety of different sensors used therewith. In a third embodiment, a single storage device (e.g., EEPROM) is associated with the entire sensor assembly, including ultrasonic transducer(s) and pressure transducer, and adapted to store and provide data relating thereto.
In a second aspect of the invention, an improved sensor housing assembly is disclosed. In one exemplary embodiment, the housing assembly comprises first and second housing elements which are fabricated from a low cost polymer and which include recesses containing the ultrasonic and pressure transducer elements, respectively. The first housing element is adapted to removably receive the second such that the active faces of the ultrasonic and pressure transducer elements are substantially aligned when the housing elements are assembled, and the second housing element (and associated pressure transducer with EEPROM) can be readily disposed of and replaced by the user when required without having to replace or dislocate the first housing element. In a second embodiment, the first housing element is also made optionally removable from the sensor assembly such that the user may optionally replace just the pressure transducer/EEPROM, the ultrasonic transducer(s), or both as desired.
In a third aspect of the invention, an improved system for measuring one or more physiologic parameters of a living subject is disclosed. In one embodiment, the physiologic parameter measured comprises arterial blood pressure in the radial artery of a human being, and the system comprises the aforementioned sensor assembly having at least one ultrasonic transducer capable of generating and receiving ultrasonic signals, a pressure transducer capable of measuring the pressure applied to its active surface, and a storage device associated therewith; a local controller assembly in data communication with the sensor assembly further including an applanation/lateral device and controller, and a remote analysis and display unit having a display, signal processor, and storage device in data communication with the local controller assembly. Calibration and other data pertinent to the sensor assembly which is stored in the storage device (e.g., EEPROM) of the sensor assembly is read out of the EEPROM and communicated to the analysis and display unit, wherein the processor within the unit analyzes the data according to one or more algorithms operating thereon. Signal processing circuits present within the local controller assembly are also used to analyze electrical signals and data relating to the operation of the sensor assembly.
In a fourth aspect of the invention, an improved circuit used in effectuating the calibration of the transducer element(s) of the aforementioned sensor array are disclosed. In one exemplary embodiment, the circuit comprises an analog circuit having a transducer element, a span TC compensation resistor Ra, analog-to-digital converter (ADC), digital-to-analog converter (DAC), and operational amplifiers. The voltage output of the pressure transducer (bridge) is input to a first stage instrumentation amplifier which amplifies the transducer output signal. The amplified output is input to a second stage amplifier, along with the output of the DAC, which is subtracted from the signal. The output of the second stage amplifier represents the temperature compensated, zero offset output signal of the circuit. The DAC converts a digital signal derived from the system processor to compensate the output for the offset of the bridge, as well as the temperature coefficient of the offset. The ADC is used to measure the bridge voltage, which varies with temperature by virtue of span compensation resistor Ra. Resistor Ra has a near zero TC, while the bridge itself has a positive TC. Thus, the bridge voltage varies with temperature, and can be correlated to the offset variation with temperature. In a second embodiment, the DAC and second amplifier are omitted, and replaced by a high resolution ADC. The converter must have the dynamic range and signal to noise ratio to measure the large output swing from the instrumentation amplifier. This is true, because the output now contains both the signal, and the offset error, and offset TC error. In this embodiment, the bridge voltage still varies with temperature and is digitized by the ADC after amplification, but all the compensation is handled by digital signal processing in the system processor.
In a fifth aspect of the invention, an improved method of operating a disposable sensor in conjunction with its host system is disclosed. In one embodiment, the method comprises storing at least one data field within the aforementioned storage device of the sensor, connecting the sensor to a host system, and determining the compatibility of the sensor with the host device based at least in part on the at least one data field. In a second embodiment of the method, the sensor is operated in order to obtain data from at least one living subject; this data is then stored within the sensor and/or host system in order to provide a retrievable record of the operation of the sensor and of the specific patient tested.
In a sixth aspect of the invention, an improved method of calibrating a transducer element used within a blood pressure monitoring device is disclosed. The method comprises providing a transducer element having a predetermined operating response and associated storage device; determining the operating response for the transducer; determining a plurality of calibration parameters based on the determined operating response; storing data representative of the calibration parameters within the storage device; and calibrating the transducer during operation based at least in part on the stored calibration parameters. In one embodiment, the transducer element comprises a silicon strain beam pressure transducer and the calibration parameters comprise reference voltage and temperature values, linearity, sensitivity, and shunted resistor values calculated using a series of predetermined functional relationships. These calibration parameters are stored in the EEPROM previously described at time of manufacture. When used during normal operation, the output of the pressure transducer is calibrated by the host system using the pre-stored calibration parameters taken directly from the EEPROM during each individual use.
In a seventh aspect of the invention, an improved method of ensuring the condition of limited (e.g., single) use components within a blood pressure monitoring device is disclosed. The method generally comprises providing a blood pressure monitoring device including a removable sensor assembly; measuring at least one parameter of a living subject using the device and sensor assembly to obtain first data; storing the first data relating to the at least one parameter; measuring the at least one parameter at a second time to obtain second data; comparing the stored first data to the second data using a predetermined criterion; and disabling the blood pressure measuring device if the criterion is not satisfied. In one embodiment, the sensor assembly comprises a pressure transducer and one or more ultrasonic transducers, which are collectively used to gather parametric data relating to the blood pressure within the radial artery of the subject. The parametric data is stored within the EEPROM, and compared with subsequent measurements taken with the same device using a comparison algorithm. In this fashion, significant differences between the parametric data obtained in successive readings is detected, which indicates that the caregiver has used the device on different patients. If certain acceptance criteria are exceeded, the system generates a disable signal which prevents completion of the analysis and display of the current measurement, as well as any subsequent measurements, until the pressure transducer (and optionally ultrasonic transducers) is/are replaced.